Priority is claimed to Patent Application Numbers 2000-77746, filed in the Republic of Korea on Dec. 18, 2000 and 2001-4035 filed in Republic of Korea on Jan. 29, 2001, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a semiconductor light-emitting device and a method for fabricating the same, and more particularly, to a GaN based Group III-V nitride semiconductor light-emitting device and a method for fabricating the same.
2. Description of the Related Art
Compound semiconductor based light-emitting diodes or laser diodes capable of emitting short-wavelength visible light are widely known. In particular, a light-emitting device (light-emitting diode) or laser diode fabricated using a Group III nitride semiconductor has received considerable attention because the Group III nitride semiconductor is a direct transition type material (direct bandgap material) emitting blue light at high efficiencies by the recombination of electrons and holes.
Referring to FIG. 1, a conventional light-emitting diode (LED) based on GaN based III-V nitrides includes an n-type GaN layer 12 on a sapphire substrate 10. The n-type GaN layer 12 is divided into first and second regions R1 and R2. The first region R1 has a larger width then the second region R2 and is not affected by etching after having been formed. Meanwhile, the second region R2 is thinner than the first region R1 because it is affected by etching after having been formed. As a result, there exists a step between the first and second regions R1 and R2 of the n-type GaN layer 12. An active layer 16, a p-type GaN layer 18, and a light-transmitting p-type electrode 20 are sequentially formed on the first region R1 in the n-type GaN layer 12. A pad layer 22 for use in bonding in a packaging process is formed on the light-transmitting p-type electrode 20. An n-type electrode 14 is formed in the second region R2 of the n-type GaN layer 12.
In FIG. 2, a conventional GaN based III-V nitride semiconductor laser diode in which n-type and p-type electrodes are arranged to face the same direction, and a ridge is formed in a region where the p-type electrode is formed, is shown. In the semiconductor laser diode, In particular, referring to FIG. 2, an n-type GaN layer 12, which is divided into first and second regions R1 and R2, is formed on a sapphire substrate 10. The first region R1 is wider and thicker than second region R2 so that there exists a step between the first and second regions R1 and R2. An n-type electrode 14 is formed in the second region R2 of the n-type GaN layer 12. An n-type AlGaN/GaN layer 24, an n-type GaN layer 26, and an InGaN layer 28 acting as an active layer, for which the refractive index increasingly higher in the upward direction, are sequentially formed on the first region R1 of the n-type GaN layer 12. A p-type GaN layer 30, a p-type AlGaN/GaN layer 32, and a p-type GaN layer 36, for which the refractive index is increasingly lower in the upward direction, are sequentially formed on the InGaN layer 28. The p-type AlGaN/GaN layer 32 has a ridge (or rib) at the center thereof, and the p-type GaN layer 36 is formed on the ridge of the p-type AlGaN/GaN layer 32. The entire surface of the p-type AlGaN/GaN layer 32 is covered with a passivation layer 34. Here, the passivation layer 34 extends to the p-type GaN layer 36 such that the current threshold is reduced. That is, the passivation layer 34 covers both edges of the p-type GaN layer 36. A p-type electrode 38 is formed on the passivation layer 34 in contact with a top surface of the p-type GaN layer 36, which is not covered by the passivation layer 34.
For a conventional light-emitting diode or laser diode based on a GaN based III-V nitride semiconductor in which the n-type and p-type electrodes are arranged to face the same direction, a bonding process with two wires should be performed on the same plan in a packaging process. Thus, the packaging process is complex and increases time consumption. The n-type electrode is formed in a deeply etched region so that a large step exists between the n-type and p-type electrodes, thereby increasing failure in packaging processes. As described with reference to FIGS. 1 and 2, in terms of the structure of the second region R2 of the n-type GaN layer 12, the n-type GaN layer 12 is etched to form the second region R2, for the light-emitting diode of FIG. 1, after the formation of the p-type electrode 20 or the p-type GaN layer 18, and for the laser diode of FIG. 2, after the formation of the p-type AlGaN/GaN layer 32. In other words, to form the h-type electrode 14 on the second region R2, an additional photolithography process is required, thereby increasing the manufacturing time of light-emitting devices.
FIG. 3 shows another conventional GaN based III-V nitride semiconductor laser diode in which an n-type electrode and a p-type electrode are arranged to face opposite directions with an active layer therebetween. An n-type GaN layer 12, an n-type AlGaN/GaN layer 24, an n-type GaN layer 26, an InGaN layer 28 acting as an active layer, a p-type GaN layer 30, a p-type AlGaN/GaN layer 32, and a p-type GaN layer 36, a passivation layer 34, and a p-type electrode 38 are sequentially formed on a silicon carbide (SiC) substrate 10a (or a gallium nitride (GaN) substrate). An n-type electrode 14a is formed on the bottom of the SiC substrate 10a. 
In general, the current threshold and the lasing mode stability for laser emission in semiconductor laser diodes are closely associated with temperature, and all quantal properties degrade as the temperature rises. Therefore, there is a need to dissipate heat generated in an active layer during laser emission to prevent a temperature rise in the laser diode. For the conventional GaN based III-V semiconductor laser diode, the substrate has a very low thermal conductivity (about 0.5 W/CmK for sapphire) so that the heat is dissipated mostly through the ridge. However, heat dissipation through the ridge is limited so that a temperature rise in laser diodes cannot be prevented effectively, thereby lowering the properties of devices.
For the conventional semiconductor laser diode shown in FIG. 2, it has been intended to dissipate heat generated in the active layer by applying a flip chip bonding technique, as illustrated in FIG. 4.
In particular, referring to FIG. 4, reference character A denotes the inverted conventional GaN based III-V semiconductor laser diode shown in FIG. 2. Reference numeral 40 denotes a submount, reference numerals 42a and 42b denote pad layers, reference numerals 44a and 44b denote first and second thermal conductive layers connected to an n-type electrode 14 and a p-type electrode 38, respectively, of the semiconductor laser diode A. Reference character M denotes a stack of material layers corresponding to the material layers 24 through 34 of FIGS. 2 and 3 stacked between the n-type GaN layer 12 and the p-type electrode 38.
As described above, heat dissipation efficiency can be improved by bonding a semiconductor laser diode to a separate heat dissipating assembly. However, bonding between the laser diode and the heat dissipating assembly increases the overall processing time. In addition, such a bonding process needs to follow a fine alignment between the semiconductor laser diode and the heat dissipating assembly, so that failure is more likely to occur, thereby lowering yield.
For example, assuming that the yield is 70%, about 4,000 pieces of laser diodes per wafer are obtained. A bonding time required for flip-chip bonding of all the laser diodes amounts about 20 hours (about 0.3 minutes each).
To solve the above-described problems, it is a first object of the present invention to provide a GaN based III-V nitride semiconductor light-emitting device which a photolithography process applied to form electrodes and a package process are simplified with reduced manufacturing time and low failure.
It is a second object of the present invention to provide a method for fabricating a GaN based III-V nitride semiconductor light-emitting device.
To achieve the first object of the present invention, there is provided a light-emitting device comprising: an active layer in which light is emitted; first and second electrodes arranged facing each other around the active layer; a first compound semiconductor layer formed between the active layer and the first electrode; a second compound semiconductor layer, opposite to the first compound semiconductor layer, formed between the active layer and the second electrode; and a high-resistant substrate formed on the bottom of the first compound semiconductor layer while being partially removed to allow an electrical contact between the first compound semiconductor layer and the first electrode.
It is preferable that a via hole exposing the bottom of the first compound semiconductor layer is formed in the high-resistant substrate, and the first electrode contacts the first compound semiconductor layer through the via hole. The first electrode is comprised of an ohmic contact layer covering a region of the first compound semiconductor layer exposed through the via hole of the high-resistant substrate, and a thermal conductive layer formed on the ohmic contact layer.
It is preferable that the high-resistant substrate covers only a portion of the bottom of the first compound semiconductor layer, and the first electrode contacts a part of or the entire first compound semiconductor layer. Preferably, the high-resistant substrate is a sapphire substrate. Preferably, both the first and second electrodes are formed of a light-transmitting material. Preferably, the first (or second) electrode is formed of a light-reflecting material and the second (or first) electrode is formed of a light-transmitting material. It is preferable that the light-emitting device further comprises a pad layer partially or fully covering the second electrode. It is preferable that the light-emitting device further comprises a pad layer partially or fully covering the first electrode. It is preferable that the first compound semiconductor layer is an n-type or undoped GaN based III-V nitride compound semiconductor layer. It is preferable that the second compound semiconductor layer is a p-type GaN based III-V nitride compound semiconductor layer. The active layer is preferably an InxAlyGa1xe2x88x92xxe2x88x92yN layer having, more preferably, a multi-quantum well (MQW) structure, where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and x+yxe2x89xa61.
In another embodiment, the present invention provides a light-emitting device comprising: a high-resistant substrate; first and second electrodes arranged with the high-resistant substrate therebetween; and a material layer formed for lasing between the high-resistant substrate and the second electrode, wherein a region of the high-resistant substrate is removed, and the first electrode contacts the material layer through the removed region of the high-resistant substrate.
It is preferable that the material layer for lasing comprises: a resonator layer; first and second cladding layers arranged with the resonator layer therebetween; first and second compound semiconductor layers arranged on the respective first and second cladding layers; and a passivation layer formed between the second cladding layer and the second electrode in contact with a region of the second compound semiconductor layer in a symmetrical manner, wherein the bottom of the first compound semiconductor layer contacts the first electrode through the removed region of the high-resistant substrate. It is preferable that the resonator layer comprises: an active layer in which lasing occurs; a first waveguide layer formed between the active layer and the first cladding layer; and a second waveguide layer formed between the active layer and the second cladding layer. It is preferable that a via hole exposing the bottom of the first compound semiconductor layer is formed in the high-resistant substrate, and the first electrode contacts the first semiconductor compound layer through the via hole. It is preferable that the high-resistant substrate covers only a region of the bottom of the first compound semiconductor layer, and the first electrode contacts a part of or the entire of the first compound semiconductor layer. The active layer is preferably an InxAlyGa1xe2x88x92xxe2x88x92yN layer having, more preferably, a MQW structure, where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and x+yxe2x89xa61.
In another embodiment, the present invention provides a light-emitting device comprising: an active layer in which light is emitted; first and second material layers with the active layer therebetween, the first and second material layers are for inducing laser emission in the active laser by lasing; a first electrode formed in contact with the lowermost layer of the first material layers; a second electrode formed in contact with the uppermost layer of the second material layers in a restricted manner; and a heat-dissipating element displaced in contact with the lowermost layer of the first material layers for effective heat dissipation.
It is preferable that the heat-dissipating element is a thermal conductive layer, and the thermal conductive layer contacts a region of the lowermost layer of the first material layers while a substrate is present on the remaining region of the lowermost layer of the first material layers. It is preferable that the thermal conductive layer contacts the region of the lower most layer of the first material layers through a via hole formed in the substrate. In this case, a dent extending into the lowermost layer of the first material layers may be formed along with the via hole in the substrate. The via hole may be formed in a region of the substrate aligned with the first electrode. A plurality of via holes may be formed in the substrate. It is preferable that the via hole extends past the lowermost layer of the first material layers. It is preferable that a region of the lowermost layer of the first material layers is etched by a predetermined thickness so that there exists a step between the region of the lowermost layer of the first material layers on which the substrate is present and the etched region of the lowermost layer where the substrate is not present. Preferably, a portion of the thermal conductive layer indirectly contacts the lowermost layer of the first material layers by the dent. In this case, a plurality of dents may be formed in the substrate, and a via hole extending past the lowermost material layer may be additionally formed in the substrate.
It is preferable that the thermal conductive layer comprises at least one selected from the group consisting of gold (Au), silver (Ag), copper (Cu), nickel (Ni), and indium (In).
To achieve the second object of the present invention, there is provided a method for fabricating a light-emitting device, the method comprising: (a) sequentially forming a first compound semiconductor layer, an active layer, and a second compound semiconductor layer, which are for inducing light emission, on a high-resistant substrate; (b) forming a light-transmitting conductive layer on the second compound semiconductor layer; (c) etching a region of the high-resistant substrate to expose the first compound semiconductor layer; and (d) forming a high-shielding conductive layer to cover the exposed region of the first compound semiconductor layer. Preferably, step (c) comprises: polishing the bottom of the high-resistant substrate; and exposing the bottom of the first compound semiconductor layer by etching the region of the high-resistant substrate. Preferably, the high-resistant substrate is a sapphire substrate. Preferably, the bottom of the high-resistant substrate is polished by grinding or lapping. Preferably, the high-resistant substrate is dry etched. In this case, a predetermined region to be a via hole or the remaining region of the high-resistant substrate may be etched. Preferably, the light-emitting device fabrication method further comprises forming a pad layer on the light-transmitting conductive layer.
In one embodiment, the present invention provides a method for fabricating a light-emitting device, the method comprising: (a) sequentially forming a first compound semiconductor layer, an active layer, and a second compound semiconductor layer, which are for inducing light emission, on a high-resistant substrate; (b) forming a light-reflecting conductive layer on the second compound semiconductor layer; (c) etching a region of the high-resistant substrate to expose the first compound semiconductor layer; and (d) forming a light-transmitting conductive layer to cover the exposed region of the first compound semiconductor layer. It is preferable that step (c) comprises: polishing the bottom of the high-resistant substrate; and exposing the bottom of the first compound semiconductor layer by etching the region of the high-resistant substrate.
In another embodiment, the present invention provides a method for fabricating a light-emitting device, the method comprising: (a) forming a material layer for lasing on a high-resistant substrate; (b) forming a first electrode on the material layer; (c) etching a region of the high-resistant substrate to expose a region of the material layer; and (d) forming a second electrode on the bottom of the high-resistant substrate to cover partially or fully the exposed region of the material layer. It is preferable that step (a) comprises: sequentially forming a first compound semiconductor layer, a first cladding layer, a resonator layer, a second cladding layer, and a second compound semiconductor layer on the high-resistant substrate; forming a mask pattern on the second compound semiconductor layer to cover a predetermined region of the second compound semiconductor layer; sequentially patterning the second compound semiconductor layer and the second cladding layer using the mask pattern as an etch mask, the second cladding layer into a rigid form; removing the mask pattern; and forming a passivation layer on the second cladding layer patterned into the ridge form, in contact with a region of the patterned second compound semiconductor layer. It is preferable that step (c) comprises: polishing the bottom of the high-resistant substrate; and exposing the bottom of the first compound semiconductor layer by etching the region of the high-resistant substrate. It is preferable that the high-resistant substrate is a sapphire substrate. Preferably, the high-resistant substrate is dry etched. It is preferable that the high-resistant substrate is etched to form a via hole through which the bottom of the first compound semiconductor layer is exposed. It is preferable that step (d) comprises: forming an ohmic contact layer on the bottom of the high-resistant substrate to cover partially or fully the exposed region of the material layer; and forming a thermal conductive layer on the ohmic contact layer.
According to the light-emitting device and the method for fabricating the same according to the present invention, the simplified bonding process reduces bonding failure, and the simplified photo and etching process makes the overall manufacture of devices easier and reduces the manufacturing time consumption.